(1) Field of the Invention
This invention relates to a change speed apparatus for a tractor comprising planetary reduction means including a sun gear mounted on a main shaft rotatable by engine output, planet gears meshing with the sun gear, and an internal gear meshing with the planet gears, frictional stepless change speed means including a driven disc operatively connected to the internal gear, and a plurality of drive discs contacting an outer periphery of the driven disc and adapted to rotate on axes normal to a rotational axis of the driven disc, and a gear mounted on the main shaft to drive the frictional stepless change speed means, the driven disc being axially shiftable to steplessly change the revolving speed of the planet gears corresponding to the travelling speed of the tractor.
(2) Description of the Prior Art
To steplessly change the travelling speed, the above change speed apparatus utilizes a mechanical construction which is less expensive than a hydraulic stepless change speed mechanism combining varied hydraulic elements. Moreover, the change speed apparatus of this invention utilizes a planetary reduction means which despite its compactness is adapted to provide large reduction, and therefore the frictional stepless change speed means is operable in high speed ranges suited for high transmission efficiency. However, in a known apparatus as disclosed in U.S. patent application Ser. No. 70,461, filed Aug. 28, 1979, now abandoned, of the present applicants, for example, the planetary reduction means and the frictional stepless change speed means are disposed away from each other and the internal gear of the former is operatively connected to the driven disc of the latter through a separately provided interlocking means comprising a shaft and gear. Such an apparatus has disadvantages of lacking in compactness and of complicated and costly construction requiring many constituent elements. Another such prior art construction is disclosed in Federal Republic of Germany laying-open gazette No. 2,207,033.
A further disadvantage of the known apparatus is that their construction does not allow the driven disc to have a large diameter. It is therefore impossible to arrange many drive discs on the periphery of the driven disc; the driven disc has to be rotated by few drive discs, each dealing with a heavy torque load, and slips can occur easily between the drive discs and the driven disc.